The present invention relates to an acceleration sensor for detecting an acceleration, and more particularly to an acceleration sensor for detecting an acceleration, based on a sliding vibration produced by application of acceleration.
An acceleration sensor is installed in equipment, and monitors an abnormal condition of the equipment by detecting an acceleration and vibration of the equipment. For example, the acceleration sensor is used to prevent errors in reading and writing data that result from vibration and shock in a hard disk drive, to prevent hand shaking in a video camera, to actuate an air bag in a vehicle, etc.
With a reduction in size and an improvement of the performance of equipment in which an acceleration sensor is to be installed, there has been a demand for the development of a small-sized, high-performance acceleration sensor capable of being mounted on a surface of the equipment. As such a small-sized acceleration sensor, an acceleration sensor using a piezoelectric element has been conventionally put into practice. Disclosed examples of such an acceleration sensor include an acceleration sensor that detects an acceleration by using a deflection of a piezoelectric single crystal (Japanese Patent Application Laid-Open No. 11-211748/1999, etc.), and an acceleration sensor that detects an acceleration by using a deflection of piezoelectric ceramic (Japanese Patent Application Laid-Open No.6-273439/1994, etc.). Moreover, a packaging method that enables compact housing of an acceleration sensor has been proposed (Japanese Patent Application Laid-Open No. 9-318650/1997, etc.).
According to the acceleration sensors using a deflection of a piezoelectric single crystal or a deflection of piezoelectric ceramic as mentioned above, by increasing the deflection to increase the stress, the detection sensitivity can be improved. Therefore, in order to improve the detection sensitivity for high performance, the mass needs to be increased to produce a larger deflection, resulting in a problem that the acceleration sensor becomes heavier and larger in size. On the other hand, when the piezoelectric element is made thicker, it does not easily deflect and causes a problem of a lowering of the detection sensitivity. Then, for an improvement of the detection sensitivity, there have been proposals to make the piezoelectric element thinner, to stick two pieces of extremely thin piezoelectric elements together, etc, but such proposals are associated with problems that the fabrication process is complicated and the cost is increased.
Therefore, the applicant of the present invention proposed acceleration sensors capable of detecting an acceleration with good sensitivity by a small-sized structure (Japanese Patent Application Laid-Open No. 2000-97707 and Japanese Application No.12-131714/2000). Such an acceleration sensor comprises a vibrator and a weight section which is connected to the vibrator and supported at a position different from the position of the center of gravity of an assembly of the vibrator and weight section, and finds the magnitude of an applied acceleration by detecting the amount of characteristic (sliding vibration) of the vibrator corresponding to an angular moment produced at the weight section by application of acceleration.
FIG. 1 is an explanatory view illustrating the detection principle of this acceleration sensor. The acceleration sensor includes a vibrator 100, a weight section 200 and detection section 300 respectively connected to the vibrator 100. The weight section 200 is supported at a support point S, and the position of this support point S is different from the position of the center of gravity G of the vibrator 100 and weight section 200. When an acceleration in one direction (the direction of a void arrow in FIG. 1) is applied to such an acceleration sensor, an angular moment (arrow A in FIG. 1, size MLa (where M: the mass of the weight section 200, L: the length from the support point S to the center of gravity of the weight section 200, a: the applied acceleration)) about the support point S is produced. This angular moment causes a sliding vibration of the vibrator 100 (arrow B in FIG. 1). The detection section 300 detects a signal resulting from the sliding vibration corresponding to such an angular moment. Since the size of the angular moment is proportional to the magnitude of acceleration to be detected, the acceleration can be detected by detecting this signal.
Besides, FIG. 2 is a perspective view showing one example of the structure of such an acceleration sensor. An acceleration sensor 50 comprises a rectangular parallelepiped vibrator 51 formed of a single crystal piezoelectric body, a long rectangular parallelepiped weight section 52 and a flat rectangular parallelepiped substrate 53. Here, although the vibrator 51 is formed of a single crystal piezoelectric body, needless to say, there is no problem even if the vibrator 51 is made of other piezoelectric body such as piezoelectric ceramics. Others are the same as below. The vibrator 51 is bonded to one end portion of the weight section 52 through an adhesive layer 54. An end portion of the vibrator 51, which faces the substrate 53, is substantially divided into two regions in its longitudinal direction, and electrodes 55 are formed on the respective front surfaces. Moreover, a pattern of electrodes 56 is formed on the front surface of the substrate 53 (a face facing the vibrator 51), and the electrodes 55 and 56 are bonded together through an adhesive layer 57.
In the case where the acceleration sensor 50 having such a structure is bonded to a specimen, when an acceleration (the void arrow direction in FIG. 2) in one direction (the width direction) is applied, an angular moment about the support point is produced at the weight section 52 by the positional difference between the center of gravity of the weight section 52 and the support point, and sliding vibrations of different orientations in the width direction are produced in both of the divided regions of the vibrator 51. Then, by drawing a voltage resulting from these sliding vibrations from the electrodes 55 via the electrodes 56 of the substrate 53 and by amplifying and detecting the voltage signal, the acceleration is detected.
Besides, the applicant of the present invention has also proposed an acceleration sensor constructed by dividing a face of the vibrator 51 on the weight section 52 side instead of a face on the substrate 53 side as described above.
The present inventor et al. are pursuing the development and improvement of such an acceleration sensor that has a small-sized structure and a high detection sensitivity without increasing the size of the vibrator itself to achieve high performance because it detects sliding vibration instead of deflective vibration.
In such an acceleration sensor, when the formation pattern of the electrodes 56 on the substrate 53 shown in the structure of FIG. 2 is asymmetrical or when the respective electrodes 56 have different thickness, the vibrator 51 inclines and causes a problem that the detection sensitivity varies.
Further, even when a resonance frequency of the vibrator 51 itself is set out of an operating frequency band of the acceleration sensor 50, if a resonance frequency of the substrate 53 is included within the operating frequency band, a signal of the resonance frequency of the substrate 53 is received, which causes a problem that flat detection sensitivity characteristics can not be obtained within the operating frequency band.
An object of the present invention is to provide an acceleration sensor capable of reducing variation in the detection sensitivity and progressing the flatness of the detection sensitivity within an operating frequency band by improving a previously proposed acceleration sensor.
An acceleration sensor according to the first aspect of the present invention is an acceleration sensor comprising a vibrator subject to a sliding vibration and a weight section connected to the vibrator and supported at a position different from a position of a center of gravity of an assembly of the vibrator and weight section, for detecting an angular moment about the support point, which is produced at the weight section by application of acceleration, as sliding vibration with the vibrator, wherein the acceleration sensor comprises a rectangular substrate having a plurality of electrodes electrically connected to the vibrator, a formation pattern of the plurality of electrodes is symmetrical about an axis parallel to at least one side of the substrate, and the plurality of electrodes have substantially equal thickness. Since the formation pattern of the electrodes on the substrate is symmetrical and the respective electrodes have substantially equal thickness, the vibrator that is bonded to this substrate does not incline and the detection sensitivity does not vary, thereby improving the detection sensitivity characteristics.
An acceleration sensor according to the second aspect of the present invention is an acceleration sensor comprising a vibrator subject to a sliding vibration and a weight section connected to the vibrator and supported at a position different from a position of a center of gravity of an assembly of the vibrator and weight section, for detecting an angular moment about the support point, which is produced at the weight section by application of acceleration, as sliding vibration with the vibrator, wherein the weight section is provided with a plurality of electrodes electrically connected to the vibrator, a formation pattern of the plurality of electrodes is symmetrical about an axis parallel to at least one side of the weight section, and the plurality of electrodes have substantially equal thickness. Since the formation pattern of the electrodes on the weight section is symmetrical and the respective electrodes have substantially equal thickness, the vibrator that is bonded to this weight section does not incline and the detection sensitivity does not vary, thereby improving the detection sensitivity characteristics.
An acceleration sensor according to the third aspect of the present invention is an acceleration sensor comprising a vibrator subject to a sliding vibration and a weight section connected to the vibrator and supported at a position different from a position of a center of gravity of an assembly of the vibrator and weight section, for detecting an angular moment about the support point, which is produced at the weight section by application of acceleration, as sliding vibration with the vibrator, wherein the acceleration sensor comprises a rectangular substrate having a plurality of electrodes electrically connected to the vibrator, a formation pattern of the plurality of electrodes is symmetrical about an axis parallel to at least one side of the substrate, and an inclination angle of the vibrator to the substrate is not larger than 40xc2x0. Although the vibrator inclines with respect to the substrate, since the inclination angle is not larger than 40xc2x0, it is possible to limit the variation in the detection sensitivity not to be higher than around 30%.
An acceleration sensor according to the fourth aspect of the present invention is an acceleration sensor comprising a vibrator subject to a sliding vibration and a weight section connected to the vibrator and supported at a position different from a position of a center of gravity of an assembly of the vibrator and weight section, for detecting an angular moment about the support point, which is produced at the weight section by application of acceleration, as sliding vibration with the vibrator, wherein the acceleration sensor comprises a substrate for taking out a result of the detection by the vibrator, and a mechanical resonance frequency of the substrate is out of an operating frequency band of the acceleration sensor. Since the mechanical resonance frequency of the substrate is set out of the operating frequency band of the acceleration sensor, it is possible to obtain flat detection sensitivity characteristics within the operating frequency band.
An acceleration sensor according to the fifth aspect of the present invention is an acceleration sensor comprising a vibrator subject to a sliding vibration and a weight section connected to the vibrator and supported at a position different from a position of a center of gravity of an assembly of the vibrator and weight section, for detecting an angular moment about the support point, which is produced at the weight section by application of acceleration, as sliding vibration with the vibrator, wherein the acceleration sensor comprises a substrate for taking out a result of the detection by the vibrator, and a mechanical resonance frequency of an integral construction of the vibrator, weight section and substrate is out of an operating frequency band of the acceleration sensor. Since the mechanical resonance frequency of the integral construction of the vibrator, weight section and substrate is set out of the operating frequency band of the acceleration sensor, it is possible to obtain flat detection sensitivity characteristics within the operating frequency band.
An acceleration sensor according to the sixth aspect of the present invention is an acceleration sensor comprising a vibrator subject to a sliding vibration and a weight section connected to the vibrator and supported at a position different from a position of a center of gravity of an assembly of the vibrator and weight section, for detecting an angular moment about the support point, which is produced at the weight section by application of acceleration, as sliding vibration with the vibrator, wherein the acceleration sensor comprises a substrate for taking out a result of the detection by the vibrator and a cap section for covering the vibrator and weight section, and a mechanical resonance frequency of an integral construction of the vibrator, weight section, substrate and cap section is out of an operating frequency band of the acceleration sensor. Since the mechanical resonance frequency of the integral construction of the vibrator, weight section, substrate and cap section is set out of the operating frequency band of the acceleration sensor, it is possible to obtain flat detection sensitivity characteristics within the operating frequency band.
The above and further objects and features of the invention will more fully be apparent from the following detailed description with accompanying drawings.